Above-ground storage tanks are those which are supported on the ground rather than buffed or supported in elevated position. Such tanks can be of substantial size and volume and may range up to a football field in diameter.
New and rebuilt above-ground storage tanks use an environmental safety secondary containment liner in the form of a plastic membrane. The liner or membrane is usually spaced a short distance beneath the metal bottom which is supported on compacted earth above and below the liner. The membrane is designed to contain leaks to prevent ground contamination.
Unfortunately, because of the dielectric properties of the secondary containment liner, conventional and widely accepted cathodic protection methods such as those using replaceable deep anodes or distributed anodes are no longer applicable. Such systems usually use large anodes which, in any event, would not normally fit in the relatively narrow envelope between the liner and the tank bottom. The dielectric liner effectively blocks the required current flow from such anodes to the tank bottom. Accordingly, to be effective, an anode system has to be placed in the relatively narrow envelope between the liner and the tank bottom.
Many operators have chosen galvanic cathodic protection systems which use zinc or magnesium ribbon anodes. These galvanic anode ribbon systems are typically installed in parallel lengths between the membrane and the tank bottom floor. This method of cathodic protection can be an effective means of tank bottom corrosion control. However, because of the large volume of anode material required fully to protect the tank bottom, these systems have proven to be quite costly.
Also, because most storage tanks use complex and highly sensitive leak detectors, it is important that the compacted medium between the liner and tank bottom not be comprised of hydrocarbons or not be carbonaceous. Experiments have shown that conventional conductive carbonaceous backfills, which are widely used with existing impressed current systems, set off such leak detectors or otherwise render them useless. The backfill must not be an electronic conductor to avoid shorting between the anode and bottom, and yet must be capable of being compacted and supporting uniformly high compressive loads. The backfill material, however, must be an electrolytic or ionic conductor.
It is also important that the anode be generally uniformly spaced from the tank bottom and not touch the bottom. If it touches, a short occurs and the system malfunctions, or if it is not substantially uniformly spaced from the tank bottom a near short may occur resulting in non-uniform distribution of the protective current. The area beneath a large storage tank is hardly accessible, and convenient repairs are virtually impossible. It is, therefore, important to use as anode materials, components which don't themselves substantially corrode, or which don't form current blocking oxidation layers. Further, the anode and the connections to the anode should provide a thin or low profile and should also be such that the system provides a minimal cathodic protection current substantially uniformly to the entire tank bottom.
In applicant's prior U.S. Pat. No. 5,065,893 there is illustrated a tank bottom anode and system constructed as a grid from oxide or precious metal coated titanium bars and ribbons, or in one embodiment mesh strips. To fabricate such grids, the bars, ribbons or strips have to be cut to length and tack welded to each other at points of intersections. While the relatively smaller ribbon can be directed on a relatively large radius of curvature, it can't be bent to go around a comer or on a short radius of curvature and remain flat without being creased. This may adversely affect the exotic passivating coating. Thus, fabrication of a large tank bottom anode requires many steps of cutting and tack welding to create the desired grid or pattern, each, however, creating a potential point of failure. Thus, the grids or patterns available are difficult and costly to fabricate and install correctly. Moreover, the coated titanium or other metal bars, ribbon and mesh strips are quite costly. Because of the cutting and fabricating required, a good portion of the material required becomes scrap. Thus, it would be advantageous to use a less costly material which can more easily be configured into the current distribution grid or maze and which can carry more current.
Another form of tank bottom anode uses a multiplicity of loops and insulated cathodic protection cable, the center of the loops being somewhat vaguely related to the center of the tank. At spaced intervals, each cable has the insulation removed and short section or pigtail of titanium wire is connected thereto, which becomes the anode. This system has a multiplicity of connections, each being a potential point of failure, is difficult, and costly to install, and provides limited grid patterns and current density control. The many free wire ends create both an installation problem and a current distribution problem since much of the current tends to flow through the cut wire ends creating what might be termed a spotty distribution pattern.
Titanium wire has additionally been used in other cathodic protection applications. For example, it has been used as a core in a cannistered anode assembly. The wire extends axially of the tubular galvanized canister and is embedded in coke breeze. At one end, the wire is connected to the anode cable through a special connection. These canistered anodes have been sold under the trademark LIDA.
Another type of titanium wire anode has been marketed by MAGNETO-CHEMIE of Schiedam, Holland. In this type of wire anode, the titanium wire is helically coiled about a current feeding cable and electrically connected to the cable at the ends or at varying centers along the cable. The cable has partial insulation.
U.K. Patent Publication GB 2175609A discloses titanium as one element in a valve metal group from which wires may be made for use in the cathodic protection of steel in steel reinforced concrete. A wire mesh or netting is formed which is used with a main anode layer. The wire mesh serves a double function as being a connection to the main anode, and a back up in the event of failure of the main anode, which is disclosed as a graphite slab mounted on top of a beam. All of the above types of anodes have cost and/or construction limitations which add to or make more difficult the task of designing and installing cathodic protection systems for new or rebuilt above-ground storage tanks.